RE: [Freetype] Tahoma sub-pixel off in 2.1.4 and CVS?

[Turner David]

Hello,

> Hello,
> 
> I've noticed that Tahoma renders badly under my particular setup as
> compared to Windows (XFree86 4.3.0, FreeType 2.1.4 and FreeType CVS,
> fully hinted, bytecode interpreter enabled, RGB sub-pixel).
> 
> A comparison graphic is available here:
> 
> http://kayalabs.com/images/tahoma.png
> 
> The text in the first column is not antialiased; note that 
> rendering is identical with Windows XP and with FreeType.
> The text in the right column was rendered using ClearType on Windows
> XP and sub-pixel rendering with FreeType.
> 
Since monochrome text is identical, I suppose that you enabled the
bytecode interpreter. In this case, I can give you an explanation:

  - FreeType, with the bytecode interpreter turned on, doesn't
    change its hinting algorithm when generating sub-pixel optimized
    glyph images. It will produce exactly the same hinted outline,
    but will use a slightly different algorithm to generate the
    RGB bitmaps. So does LibXft on Linux, which adds filtering to
    avoid color-fringes

  - Microsoft's ClearType does use a different hinting algorithm,
    at least in the horizontal dimension, that's why the ClearType
    glyphs often appear blurry at certain sizes, when their "normal"
    anti-aliased counterparts don't. I have no details on what's
    happenning under the hood though. Any ideas are welcomed.

  - FreeType's auto-hinter does change its hinting algorithm
    when rendering sub-pixel RGB glyph images though, and this
    is the result of what you get in "ftview" when pressing the
    "l" key repeatedly. Note that I don't think that libXft uses
    this feature yet, since it depends on an experimental API
    that hasn't been completely declared public yet (i.e.
    FT_LOAD_TARGET_LCD & FT_LOAD_TARGET_LCD_V)


> In general I prefer FreeType's version to ClearType; however, 
> there are some problems.  My biggest issues are with 'A' and 'S'; 'A' 
> is too bold and 'S' is horizontally splotchy in the center.  '2' and 'K' 
> contain
> diagonals which are too thin, and 'a' and 'w' are  particularly noisy as
> well.
> 
I haven't seen your graphics, but I suppose that the problem you're
seeing come from the fact that the TrueType hints of these fonts were
not written for sub-pixel hinting at all. The artefacts you're
seeing are the result of the extreme distortions created by the
bytecode instruction. These are normally not visible with monochrome
or normal anti-aliased rendering. However, sub-pixel rendering uses
a higher resolution and makes certain defects visible.

The same is true of defects that appear in anti-aliased glyphs but
not in monochrome ones.

Hope this helps,

- David Turner
- The FreeType Project  (www.freetype.org)

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